1. Technical Field
The invention relates to the art of tire inflation systems. More particularly, the invention relates to tire inflation systems for heavy-duty vehicles such as trucks and tractor-trailers or semi-trailers, which can operate as the vehicles are moving. Still more particularly, the invention is directed to a tire inflation system which includes a tire deflation circuit that is discrete or separate from an inflation circuit, thereby enabling control of tire deflation based on specific predetermined conditions.
2. Background Art
Heavy-duty vehicles typically include trucks and tractor-trailers or semi-trailers. Tractor-trailers and semi-trailers, which shall collectively be referred to as tractor-trailers for the purpose of convenience, include at least one trailer, and sometimes two or three trailers, all of which are pulled by a single tractor. All heavy-duty vehicles that are trucks or tractor-trailers include multiple tires, each of which is inflated with a fluid or gas, such as air, to an optimum or recommended pressure. This optimum or recommended tire pressure typically is referred to in the art as the target inflation pressure or the target pressure.
However, it is well known that air may leak from a tire, usually in a gradual manner, but sometimes rapidly if there is a problem with the tire, such as a defect or a puncture caused by a road hazard. As a result, it is necessary to regularly check the air pressure in each tire to ensure that the tires are not significantly below the target pressure and thus under-inflated. Should an air check show that a tire is under-inflated, it is desirable to enable air to flow into the tire to return it to the target pressure. Likewise, it is well known that the air pressure in a tire may increase due to increases in ambient air temperature, so that it is necessary to regularly check the air pressure in each tire to ensure that the tires are not greatly above the target pressure and thus over-inflated. Should an air check show that a tire is over-inflated, it is desirable to enable air to flow out of the tire to return it to the target pressure.
The large number of tires on any given heavy-duty vehicle setup makes it difficult to manually check and maintain the target pressure for each and every tire. This difficulty is compounded by the fact that trailers of tractor-trailers or trucks in a fleet may be located at a site for an extended period of time, during which the tire pressure might not be checked. Any one of these trailers or trucks might be placed into service at a moment's notice, leading to the possibility of operation with under-inflated or over-inflated tires. Such operation may increase the chance of less-than-optimum performance and/or reduced life of a tire in service as compared to operation with tires at the target pressure, or within an optimum range of the target pressure.
Moreover, should a tire encounter a condition as the vehicle travels over-the-road that causes the tire to become under-inflated, such as developing a leak from striking a road hazard, or over-inflated, such as increasing pressure from an increased ambient air temperature, the life and/or performance of the tire may be significantly reduced if the under-inflation or over-inflation continues unabated as the vehicle travels. The potential for significantly reduced tire life typically increases in vehicles such as trucks or tractor-trailers that travel for long distances and/or extended periods of time.
Such a need to maintain the target pressure in each tire, and the inconvenience to the vehicle operator to manually check and maintain a proper tire pressure that is at or near the target pressure, led to the development of prior art tire inflation systems. In these prior art systems, an operator selects a target inflation pressure for the vehicle tires. The system then monitors the pressure in each tire and attempts to maintain the air pressure in each tire at or near the target pressure by inflating the tire when the monitored pressure drops below the target pressure. These prior art tire inflation systems inflate the tires by providing air from the air supply of the vehicle to the tires by using a variety of different components, arrangements, and/or methods. In prior art systems that are also capable of deflation, the system deflates the tire when the monitored pressure rises above the target pressure by venting air from the tires to atmosphere.
While being satisfactory for their intended functions, tire inflation systems of the prior art may experience disadvantages in certain situations. More particularly, many prior art tire inflation systems are not capable of deflation. As a result, when the air pressure in a tire increases to a level that is greatly above the target pressure due to increases in ambient air temperature, these systems are not able to reduce the pressure in the tires. As a result, such prior art tire inflation systems may allow the tires to operate in a significantly over-inflated condition, which undesirably decreases performance of the tires and in turn decreases the life of the tires.
In addition, in those prior art tire inflation systems having a deflation capability, the systems generally inflate and deflate the vehicle tires through the same components, circuit or path of pneumatic conduit, valves and the like that extend from the vehicle air supply to the tires, which is referred to herein as a circuit. Use of the same circuit for inflation and deflation functions has been achieved in the prior art by employing electronically-controlled systems that include electronically-actuated solenoid valves. With a solenoid valve, when it is desired to inflate the tires, an electronic controller actuates the valve to move the valve to a position that enables air to flow from the air reservoir to the vehicle tires. When it is desired to deflate the tires, the electronic controller actuates the valve to move the valve to a position that exhausts air from the tires to atmosphere. Such prior art deflation-capable tire inflation systems have certain disadvantages.
First, prior art tire inflation systems only maintain the inflation pressure in the tires at the target pressure, and lack the ability to accommodate an increased tire pressure based on operating conditions. More particularly, the desired target inflation pressure typically is selected by the vehicle operator based on what is known in the art as a cold inflation pressure or cold pressure, which is the inflation pressure of the tires when the vehicle remains parked. In many cases, the tire manufacturer recommends a target pressure that is at a cold pressure setting for a specific axle load.
However, as the vehicle operates and travels over-the-road, the energy and forces associated with the travel cause the temperature of each vehicle tire to increase. When the temperature of the tire increases, the air inside the tire expands. Because the volume of the tire is limited, the expansion of air causes the air pressure inside the tire to increase above the cold inflation pressure. This increased air pressure is typically referred to as the operating pressure of the tires. By way of example, the operating pressure may be about fifteen (15) pounds per square inch (psi) greater than the cold pressure of each tire in a typical heavy-duty vehicle dual-wheel configuration. In fact, the National Highway Traffic Safety Administration (NHTSA) recommends adding about 15 psi to a cold pressure setting when checking pressure while the tires are at their operating temperature. The increase to the operating pressure is desirable, as tire manufacturers typically rely on the increase to compensate for lower side wall stiffness of the tire as its temperature increases during over-the-road travel, and thus often design heavy-duty vehicle tires to provide optimum performance at the operating pressure.
Because the vehicle operator typically selects a target inflation pressure for the tires which is at the cold inflation pressure, prior art tire inflation systems inflate or deflate the tires as needed to arrive at this cold target pressure. However, as described above, as the vehicle operates, the air pressure in the tires increases from the cold pressure to the higher operating pressure. Because prior art tire inflation systems only maintain the inflation pressure in the tires at the target pressure, as the air pressure in the tires increases during vehicle operation, the systems deflate the tires down from the optimum operating pressure to the lower cold target pressure. Due to this lack of ability to accommodate an increased tire pressure based on operating conditions, prior art tire inflation systems often maintain the inflation pressure of the tires at a level that is below the optimum operating pressure, which decreases tire performance, and thus vehicle performance.
In the event that the vehicle operator attempts to prevent a prior art inflation system from deflating the tires down from the optimum operating pressure to the lower cold target pressure by selecting a target inflation pressure which is at the higher operating pressure, undesirable demands may be placed on the system. More particularly, because the operating pressure is higher than the cold pressure, the operating pressure may approach or be at a pressure level that is not available in the vehicle air supply, or which would require the vehicle air supply to be maintained at an undesirably high level. The requirement of maintaining such a pressure level in the vehicle air supply places undesirable demands on the tire inflation system, which in turn reduces the performance and/or the life of the system. As a result, it is not practical to attempt to prevent prior art systems from deflating the tires down from the optimum operating pressure to the lower cold target pressure by selecting a target inflation pressure that is at the operating pressure.
A second disadvantage of prior art tire inflation systems is that most systems which are capable of both inflation and deflation are electronically controlled, which is undesirably expensive, complex, and potentially undependable. For example, electronically-controlled systems typically involve electronically-operated solenoid valves, electronic controllers, and other electronic components, which are expensive and are often complex to install and configure. In addition, these electrical components require the use of the electrical system of the vehicle, which may be unreliable or even non-functional at times, and in turn renders the operation of the tire inflation system unreliable and potentially non-functional.
A third disadvantage of prior art tire inflation systems is that the electronic systems are not constant-pressure systems. More particularly, when the system is not performing inflation, the pneumatic conduit of the system is exhausted to atmosphere and thus does not actively monitor tire pressure. In such a system, without air pressure in the pneumatic conduit, electronic controls are employed to periodically check tire pressure, and to in turn trigger or commence inflation. Because such prior art systems are capable of only providing a periodic check of tire pressure, any inflation to bring the tires to the target pressure only takes place following the periodic check. This lack of ability of prior art systems to continuously monitor tire pressure and dynamically respond to pressure changes undesirably reduces the ability of the system to actively or quickly respond to reduced tire pressure conditions, such as in the case of an air leak. Moreover, as mentioned above, the electronic controls that are employed by prior art tire inflation systems to determine when it is necessary to trigger or commence inflation are expensive, complex, and require power from the electrical system of the vehicle, which may be unreliable.
A fourth disadvantage of prior art tire inflation systems occurs in certain pneumatically-controlled systems which are constant-pressure systems, that is, systems that maintain air pressure at all times in a pneumatic conduit extending between the vehicle air reservoir and the tires. Some of these constant-pressure systems include a wheel valve that is capable of deflation, which keeps the inflation path from the air reservoir to the tires open. As is known to those skilled in the art, when a vehicle is parked for an extended period of time, the pneumatic pressure in the air reservoir may drop or bleed down due to small air leaks that are typical in any pneumatic system. Because prior art constant-pressure systems that include a wheel valve which is capable of deflation keep the inflation path from the air reservoir to the tires open, when the pneumatic pressure in the air reservoir drops, the pneumatic pressure in the tires also drops. This pressure drop may be up to 25 psi or more, at which point the wheel valve typically closes to eliminate an even greater pressure drop.
However, when the vehicle is started up to prepare for over-the-road travel, the tire inflation system must re-inflate each tire up to or near the target pressure, which may thus involve adding about 25 psi to each one of eight or more tires. This re-inflation process typically takes a great deal of time and places repeated demands on the tire inflation system, which may reduce the life of the system. In addition, the vehicle operator may not wait for the tires to be re-inflated to the target pressure before operating the vehicle, which in turn causes the tires to be operated in an under-inflated condition until the target pressure is reached. Such operation reduces the life of the tires. As a result, it is desirable for a constant-pressure tire inflation system to optionally include a feature that would isolate the tires from the air reservoir and other components of the system when the vehicle is parked, thereby minimizing pressure loss from the tires and in turn minimizing the subsequent time and demand on the system that is required to provide significant re-inflation of the tires.
A fifth disadvantage of prior art tire inflation systems occurs in certain pneumatically-controlled, constant-pressure systems that do not include a wheel valve that is capable of deflation. More particularly, without a wheel valve that is capable of deflation, such prior art systems cannot respond to excessive increased tire pressure from an increased ambient air temperature, as described above for prior art systems that are not capable of deflation. As a result, such prior art tire inflation systems may allow the tires to operate in a significantly over-inflated condition, which undesirably decreases performance of the tires and in turn decreases the life of the tires.
As a result, there is a need in the art for a tire inflation system that overcomes the disadvantages of the prior art by providing control of the conditions under which deflation occurs, by providing a system that has the ability to accommodate an increased tire pressure due to operating conditions, does not employ electronic components and thereby is more economical, simpler, more dependable and more efficient than tire inflation systems of the prior art, and which is a constant-pressure system that is capable of deflation and optionally includes a feature that enables isolation of the tires from the air reservoir and other components of the system when the vehicle is parked to minimize pressure loss. The tire inflation system with discrete deflation circuit of the present invention satisfies this need, as will be described in detail below.